1. Field of the Invention
The present invention relates to magnetic recording media such as a cassette tape, a floppy disc, a hard disc and the like.
2. Prior Art
In the past, the recording areas in magnetic recording media such as a cassette tape, a floppy disc, a hard disc and the like, have been formed of magnetic layers. As a result, recording can be made at any position on data tracks formed on the recording media.
However, noise/overwrite from areas unnecessary for recording/reproducing is appreciable. Accordingly, the residual noise characteristic or the overwrite characteristic deteriorates.
To avoid this defect, floppy disc units have employed a method of performing DC erasing for opposite sides of a recording track in order to homogenize the magnetizing direction of the two sides of the recording track before recording or a method of performing DC erasing for the opposite sides of a recording track after recording.
However, noise cannot completely be removed from the area where DC erasing has been made.
Furthermore, floppy disc units of the type using DC erasing of the opposite sides before recording, has a disadvantage that head access time is prolonged. Floppy disc units of the type using DC erasing of the opposite sides after recording, has a disadvantage that two or more erasing heads are required separately from the recording head to thereby make the assembling structure complicated and make the cost high.
Although hard disc units can perform recording/reproducing by use of a single head, noise cannot completely be removed from the area where erasing has been made.
In the case where recording is made on recording media such as a floppy disc unit and a hard unit, the floating amount h of the recording head increases at the outermost part of the disc because of the difference in linear velocity V in the direction of the radius of the disc. Accordingly, the distance (spacing) between the disc and the recording head increases to thereby enlarge the recording electric current, Iw. Consequently, the width of an outer track is larger than that of an inner track.
Furthermore, a reproductive output difference arises between the outer part and the inner part of the disc. Factors causing the difference are considered to be (1) the linear velocity V, (2) the head floating amount h, (3) the recording current Iw, (4) the leakage magnetic field intensity Hm of the head on the disc surface, (5) the rate of magnetic flux change dMr/dt, and the like.
The reproductive output, that is, the rate of magnetic flux change dMr/dt, is influenced by the linear velocity V, as shown in FIG. 4. Accordingly, it is apparent that the reproductive output in the outer circumferential part of the disc is larger than the reproductive output in the inner circumferential part of the disc, because the linear velocity V in the outer circumferential part is larger than that in the inner circumferential part. Furthermore, the head floating amount h increases as the linear velocity V increases. Accordingly, the reproductive output decreases as the floating amount h increases.
On the other hand, the reproductive output is influenced by the magnetic wall thickness (magnetic transition width) of the track at the time of reproduction. As the magnetic transition width Wj decreases, the reproductive output increases. In general, the magnetic transition width Wj satisfies the following relation: EQU Wj.alpha.(t.multidot.Br).sup.a58 .multidot.Gh.sup.-0.54
where t represents the thickness of the magnetic film, Br represents the residual magnetic flux density, and Gh represents the magnetic head magnetic field gradient. Accordingly, it is apparent that, as the magnetic head magnetic field gradient Gh decreases, the magnetic transition width Wj increases and the reproductive output decreases.
The magnetic head magnetic field gradient Gh is influenced by the respective values of the recording current Iw and the leakage magnetic field intensity Hm. The recording current Iw is influenced by the floating amount h. The leakage magnetic field intensity Hm is influenced by both the recording current Iw and the floating amount h. Accordingly, as the head floating amount h increases, the magnetic head magnetic field gradient Gh decreases, and consequently, the magnetic transition width Wj increases.
In short, the reproductive output is proportional to the factors, such as the linear velocity V, the recording current Iw, the leakage magnetic field intensity Hm of the head on the disc surface, and the like, while inversely proportional to the floating amount h of the head.
Though considering that the floating amount of the outer part is larger than that of the inner circumferential part with respect to a magnetic disc in which recording is made with a suitable value of the recording current Iw, the reproductive output in the outer circumferential part is larger than the reproductive output in the inner circumferential part because the velocity V in the other circumferential part is larger than the velocity in the inner circumferential part. In addition, conventionally the lower limit of the reproductive output is established by reference to the innermost track because the reproductive output at the innermost track is lower than the reproductive output at the outermost track. Accordingly, the lower limit should be established to a very large value, because the reproductive output increases as the position shifts to the outer circumferential part of the disc. As the result, not only a circuit for keeping the reproductive output constant has been required but efficient recording could not be made.
Further, in hard discs there has been such a problem that, when overwritten, the bit position is displaced to thereby make the overwrite characteristic poor.
In a method of surface magnetization as shown in FIG. 12, with regard to the magnetizing level L with respect to the magnetic pattern formed on the data track DT, the reverse magnetic intensity is most strong at the boundary between bits, that is, at the magnetic wall part, and the width W thereof (magnetic transition width) is large. As the result, a sharp reproductive waveform S cannot be obtained, and, accordingly, there has been a limit in shortening the bit length. Accordingly, the limitation has caused a problem in attempts at high density recording.